1. Field of the Invention
The present invention relates to a wavelength division-multiplex system, and more specifically to a wavelength division-multiplex system having a capability of saving other wavelengths.
2. Description of the Related Art
Recently, a wavelength division-multiplex communications system (hereinafter referred to as a WDM system) has been widely used as a backbone circuit, and several years have passed since the WDM system was introduced. With an increasing circuit capacity, it has become essential to construct a WDM system free of the influence of the change in number of wavelengths multiplexed in the WDM system on the circuit containing residual wavelengths.
FIG. 1 shows the outline of the configuration of the transmission unit and the reception unit of the conventional WDM system.
A transmission unit 100 of the WDM system comprises a transponder unit (TRP unit) 110 for converting a signal of a client system into a format (an optical signal of a narrow band) which can be stored in the WDM system, an MUX unit 111 for multiplexing wavelengths, an AMP unit 112 for collectively amplifying a multiplexed optical signal, and an ATT unit 113 for adjusting the optical intensity of the transmitted optical signal. A reception unit 200 comprises an AMP unit 214 for amplifying a received wavelength division multiplex signal, a DMUX unit 215 for demultiplexing a wavelength division multiplex signal into each wavelength, and a transponder unit 216 for converting an optical signal into a client signal which is an electric signal.
In the transmission unit 100, the optical power of an optical signal whose wavelength has been converted by the transponder unit 110 is adjusted by a fixed or variable optical attenuator (ATT unit 113), and the signal is input to and multiplexed by the MUX unit 111. The wavelength division multiplex signal is collectively amplified by the AMP unit 112 for long-distance transmission. In the reception unit 200, the wavelength division multiplex signal is collectively amplified by the AMP unit 214 and demultiplexed by the DMUX unit 215 for each wavelength so that the signal can be converted into the optical power receivable by the transponder unit 16 when it is demultiplexed. Each wavelength-demultiplexed signal is converted by the transponder unit 216 into optical intensity and wavelength receivable by the client system.
FIGS. 2A through 2C are explanatory views of the operation modes of the AMP unit.
In the method for eliminating the effect of the change in number of wavelengths stored in a WDM system, the system of controlling the gain of the AMP unit has conventionally been operated by the ALC (automated constant control of output power: to maintain constant total output power) in a normal operation, and the gain control of the AMP unit is switched to the AGC (automated constant control of gain) when the number of wavelengths is changed. In this system, even if the number of wavelengths is changed, the optical levels of other wavelengths are not changed at a speed at which the gain control of the AMP unit 112 can be sufficiently performed. Therefore, the transmitting process is not affected.
That is, in the WDM system, the ATT unit controls the optical power of each wavelength input to the AMP unit 112 to be constantly maintained. Therefore, in the ALC mode shown in FIG. 2A, the output power is constant, and the optical power of each wavelength is also constant. That is, since the optical power is controlled to be constant regardless of the input power, the power of a wavelength output from the AMP unit 112 can be controlled. Furthermore, since control is performed with a target value of output power so that constant output power of the entire wavelengths can be maintained, the error of the optical power per wavelength is very small. The operation of the ALC mode is expressed by the following equationoutput power per wave(dBm)=(entire output power)−(10×log(number of wavelengths)
Since the amplification rate is constant in the AGC mode shown in 2B, the optical power of each wavelength relates to the input power only. That is,output power of wavelength(dBm)=(input power of wavelength)+amplification rate(dB)
When control to maintain a constant amplification rate is not performed with a target value of output power per wavelength, the output power per wavelength generates an error after a long time period.
Therefore, as shown in FIG. 2C, the AMP unit 112 normally operates in the ALC mode, and enters the AGC mode when there arises a change in the number of wavelengths, etc. When the change in the number of wavelengths is stopped, it enters back into the ALC mode.
Therefore, it is added or removed with an optical attenuator provided before a signal is multiplexed such that the AGC control can sufficiently work when a wavelength is added or removed. As a result, even if light is suddenly inputted, the optical attenuator can reduce it down to an ignorable level. When a wavelength is added or removed, the amount of attenuation can be stepwise adjusted, thereby suppressing optical fluctuation at a speed acceptable by the gain control by the AMP unit 112.
However, in the conventional method, when a wavelength is added or removed according to the intention of the operator of the system, the number of wavelengths can be changed without the influence on the other wavelengths. However, when the input to the AMP unit suddenly changes such as when a wavelength is suddenly lost, etc., there is a possibility that other wavelengths are affected because the changing speed of the wavelength is not slow.
FIG. 3 is an explanatory view showing the problem with the prior art.
Since the lowest level of the optical reception of the power of the output of the AMP unit 112 is determined for longer-distance transmission without a relay device, it is desired that higher output can be obtained on the transmission side. However, when the power per wavelength of the optical input to an optical fiber is high in the transmission system of the WDM technology, the optical nonlinear effect generates interference between wavelengths after the transmission of the optical fiber, thereby affecting the other wavelengths. Therefore, it is common that the power per wavelength is to be restricted on the optical fiber input side. When a wavelength is added or removed according to the intention of the operator, the optical nonlinear effect can be suppressed by controlling the input power per wavelength from the AMP unit 112 to the optical fiber by changing the gain control of the AMP unit 112 in the process of ALC (normal operation)→AGC (addition or removal of a wavelength)→ALC (normal operation). For example, when the input power to the AMP unit 112 suddenly decreases due to the disconnection of a wavelength by a fault of the transponder unit 110, etc., the number of wavelengths changes in the ALC mode during the time lag in the transition of modes even if the gain control is switched from the ALC to the AGC using any means. As a result, the optical power per wavelength appears large (in FIG. 3, a signal disconnection occurs in the dotted line portion, and the optical power of the other wavelengths is higher), and the optical nonlinear effect has an influence on the wavelengths in the operation. Actually, there is an optical margin to some extent in the system. Therefore, the nonlinear phenomenon does not occur from the fluctuation of one or two wavelengths. However, for example, if a fault affecting a number of wavelengths occurs in the system block before the AMP unit 112, and a large number of wavelengths are lost, then the level fluctuation in the remaining wavelengths refers not only to the fluctuation in the optical margin but also to the nonlinear phenomenon on the remaining wavelengths, and the remaining wavelengths are suffered an influence by the addition or removal of a wavelength.
The technology of removing the undesired influence of an optical signal disconnection is disclosed by the patent literature 1. In the patent literature 1, the power supply is duplexed so that an alarm signal for detection of a fault of the power supply is multiplexed with the main signal for transmission to the transmission line.
FIG. 4 shows, as an example of an occurrence of a conventional problem phenomenon, the fluctuation of the AMP output power when a WDM system operating with 40 wavelengths has lost its wavelengths down to 4 wavelengths by the failure of the transponder unit (TRP unit). In the AMP unit in which a wavelength is output at 3 dBm, control is performed such that 40 wavelengths are output at 19 dBm. Assume that the power supply of the transponder unit (TRP unit) suddenly becomes faulty and the device accommodating 36 wavelengths stops. Since 36 wavelengths are simultaneously stopped in this case, the light input to the MUX unit (that is, the light input to the AMP unit) has only 4 wavelengths in the ALC mode. The output of the AMP unit in this status is controlled to maintain the output at 19 dBm because it is operated in the ALC mode. That is, four wavelengths are output at 19 dBm, that is, the light is output from the AMP unit at 13 dBm per wavelength. This status continues until the AMP unit detects a lost wavelength. After the AMP unit detects the lost wavelength, it enters the AGC mode, the amplification rate indicates a standard value, and the output power per wavelength returns to the standard value. The time taken by the AMP unit to detect a lost wavelength from the fault of the TRP unit is several hundred ms through several seconds. However, when the optical power per wavelength suddenly increases, an undesired influence appears on the remaining wavelength due to the nonlinear phenomenon. The phenomenon similarly has an undesired influence on the remaining wavelengths when a wavelength interference occurs at a high speed at which the gain control cannot be performed even in the AGC mode.